Hardiness is a quality of being tough, rugged, or vigorous. Your hardiness means you're able to work all day on your family's farm, even when it's extremely hot out.
The noun hardiness comes from hardy, which was originally defined as "bold and daring in battle," from a root that means "hard." An important quality in a soldier or fighter is hardiness, the ability to keep going despite difficulties and challenges. When we talk about the hardiness of plants, we mean their resistance to disease or drought.